The present invention relates to an electrical terminal unit and more particularly, to an electrical terminal unit specially adapted for supporting electrical circuit elements or impedance elements for sensing light rays, various gases, humidity, temperatures or the like.
More specifically, when pollutants such as dust, dirt, oil, moisture, etc. adhere to a terminal unit supporting electrical circuit elements of high impedance, the electrical resistance on the surfaces of such a terminal unit is reduced allowing leakage current to flow between the electrically conductive terminal leads or lead wires thereof. The reduction of the surface resistance as described above brings about variations in the characteristics of sensing devices employing high impedance electrical circuit elements. In the present invention, the reduction in the insulation resistance between the electrically conductive terminal lead wires due to adhesion of pollutants to the terminal unit is made negligible by providing an electrically conductive member or electrode member between the terminal lead wires extending through a base or substrate of the terminal unit for electrically separating such terminal lead wires, with the electrode member being grounded.
Commonly, electrical circuit elements are surrounded by casings or the like so as to reduce the effects of the external atmosphere during use. Accordingly, various pollutants such as dust, dirt, oil particles, moisture, etc. in the air rarely adhere to the electrical circuit elements, and thus, variations in the characteristics of these electrical circuit elements due to adhesion of pollutants thereto may normally be ignored.
On the contrary, however, there are some electrical circuit elements directly exposed to the outside atmosphere during use. The outstanding types of these elements are various sensing elements, for example, gas sensors including the smoke sensors, and humidity sensors which are used at all times through direct exposure thereof to air or other gases. Most of these sensing elements have high impedances, and not only their inherent sensitivity but also the leakage current between terminal wires embedded in the base of the terminal unit supporting such sensing elements largely affects the detecting sensitivity and life of such sensing elements. More specifically, the surfaces of sensing elements in general as well as the surfaces of the sensing elements for gas and humidity are subject to soiling by dust, dirt, oil, moisture, etc., while the fixed terminal lead wires and base of the terminal unit are simultaneously soiled thereby increasing the leakage current, with consequent significant errors in the detected current.
By way of example, the present state of the art of such sensing elements will be described hereinbelow with reference to a humidity sensing element.
Humidity sensing elements are important as fundamental elements used in various fields, for example, in industrial fields such as chemistry, chemical agents, paper and food articles, in argicultural fields such as soil control and greenhouse cultivation, in medical fields such as sterilization, microbe cultivation and pharmacy, and also in control of appliances such as air conditioners, microwave ovens, electric ovens, etc. The so-called sensing portion of a humidity sensing element is a moisture responsive resistor which is arranged to sensitively respond to humidity through the variations of air resistance due to fluctuations in humidity. As is well known, metallic oxides such as Fe.sub.2 O.sub.3, Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3, NiO, MgCr.sub.2 O.sub.4, etc. are superior in water absorption properties, and the moisture responsive resistor is formed by utilizing such properties. The moisture responsive elements, which are normally exposed not only to water vapor but to an atmosphere containing various other components during use, tend to lose their inherent moisture responsive resistance properties through a variety of complicated physical and chemical processes between these other components and the various materials employed. Upon deterioration of the moisture responsive properties of the moisture responsive resistor as described above, the original humidity characteristics of this moisture responsive resistor may be restored by eliminating almost all of the absorbed components as well as the oil component from the moisture responsive resistor through heating thereof at temperatures, for example, higher than 400.degree. C. Therefore, by providing a heater or heating element for cleaning purposes in the vicinity of the moisture responsive resistor which is energized, for example, at the time of each use for heating the moisture responsive resistor and thus removing the pollutants adhering to the surface thereof through burning the resistor can be repeatedly used semipermanently. Although the problem related to the deterioration of the moisture responsive characteristics of the moisture responsive resistor may be solved by the employment of a heater as described above, it is still impossible to prevent insulation leakage due to adhesion of dust, dirt, oil, etc. at the base and terminal portions of the terminal unit to which the moisture responsive resistor is connected. In other words, although the properties of the moisture responsive resistor may be repeatedly restored in the manner as described above, it is extremely difficult to remove dust, dirt, oil and the like from the base of the terminal unit and the terminal lead wires supporting this resistor through heat cleaning on the like to restore the insulation resistance thereof, due to complications in structure and increase in size of the sensing device.
Referring to FIG. 1 showing one example of the construction of a conventional humidity sensing device, the moisture responsive resistor S is connected, by lead wires Sl to corresponding ends of terminal lead wires ta and tb which are secured to an insulating base or substrate B in a spaced relation from each other, while the other ends of the terminal lead wires ta and tb which extend through the base B are connected to each other through a signal source V and a detecting resistor R. In the arrangement as described above, the moisture responsive resistor S may be cleaned by providing a heating element nearby as stated earlier, but the oil, dust, moisture, etc. adhering between the terminal lead wires ta and tb, and also on the base B can not be readily removed from the constructional point of view.